Depression of Serotonin (S) uptake by pulmonary endothelium is an early and reversible alteration of lung function due to high partial preessures of O(2). Preliminary studies indicate that high partial pressures of O(2) depress uptake by inhibiting transcellular transport of S by endothelial cells. Tranbscellular transport of S is energy and sodium dependent, and involves a carrier mediated cotransport process that is linked to Na+-K+ ATPase. As such, there are three possible mechanisms which can be advanced to explain depression of S transport by high O(2) tensions: (1) inhibition of Na+-K+ ATPase activity; (2) inhibition of the generation and/or delivery of high energy intermediates needed for transcellular transport; and (3) alterations in the structure and/or transport characteristics of the membrane carrier. To determine the mechanism responsible for hyperoxic depression of uptake and to determine whether it is unique to O(2) induced endothelial cell injury, we will measure S uptake, Na+-K+ ATPase activity, cellular ATP content, lipid peroxide generation, and the kinetic constants (Km and Vmax) of the membrane carrier for S in calf aortic and pulmonary endothelial cells, vascular smooth muscle cells and lung fibroblasts that have been exposed to hyperoxia. Once the mechanism is determined, attention will be focused on ways in which to protect against this mechanism of injury. Protection will be ways in which to protect against this mechanism of injury. Protection will be attempted by the addition of vitamin e, glutathione, superoxide dismutase, or endotoxin to the culture medium of the cells. An awareness of the specific mechanism by which O2 interferes with the metabolic function of pulmonary endothelial cells allows for the rational development of ways in which to protect against this injury. This is of particular importance because the pulmonary endothelial cell is a critrical target cell in O2 toxicity. Information gained about the response of endothelial cells to high partial pressures of O2 may be applicable to O2-induced injury in other cells and may help explain many of the physiological consequences of O(2) toxicity in other organs. Finally, information gained about the response of pulmonary endothelial cells to high partial pressures of O2 may allow broader insights into mechanisms of endothelial cell injury and repair in general.